850Mhz on the core is extremely optimistic. Most 4870s cant get there and even with a voltage bump it may be difficult. Also, you'll have to lower your mem clocks to raise the GPU clocks, especially since you have a 1GB version which is not a great clocker on memory.
I say start from stock clocks and start bumping your core and mem by 5Mhz at a time, that will give you a balanced core/memory overclock. Then keep testing for stability with ATI Tool & Furmark. Once you hit a wall and are brave enough to increase voltage via hardmods (solder or pencil) look in the Articles section for guides. Alternatively, you may be able to flash the BIOS with a higher voltage thus allowing you to overclock further. Sadly, there are no voltage softmods since you have a non-refence card.